Connectors used as connecting terminals of electrical circuits have been conventionally formed from thermosetting resins such as phenol resin, but in these years, thermoplastic resins having high moldability have come to be used instead of the thermosetting resins. Further, the connectors have come to be used not only under mild conditions such as conditions within OA machine, for example, as connectors of electric devices, but also under extremely severe conditions such as those within an automotive engine room. Therefore, the connectors used under the severe conditions such as those within an automotive engine room are required to have extremely high heat resistance. In addition, the connectors are required to have such new features that they are hardly changed in their properties even after repeated heating due to engine heating and that they are hardly changed in their properties even if they are brought into contact with water, oil, etc.
Furthermore, under the world-wide proposal of efficient utilization of petroleum energy, it has been studied to make more lightweight automobiles for the purpose of reducing fuel cost of automobiles and it has been also studied to develop small sized automotive parts.
As the thermoplastic resins for connectors, there have been conventionally used poypropylene (PP), aliphatic polyamide (NY) such as polycapramide (nylon 6, NY 6) or polyhexamethylene adipamide (nylon 66, NY 66), polyphenylene ether (PPE) and acrylonitrile/butadiene/styrene resin (ABS resin).
In these thermoplastic resins, PP is insufficient in heat resistance and low in rigidity as the resin for a connector used under severe conditions. Moreover, PP has such a problem that the rate of crystallization is slow. PPE has a certain level of heat resistance, but it is low in chemical resistance, particularly oil resistance, so that PPE is unsuitable as a resin for a connector used near machines such as an engine room. In addition, PPE has such a problem that the moldability thereof is bad because of its low flowability. The ABS resin is also unsuitable as a resin for a connector used under severe conditions in viewpoints of heat resistance, chemical resistance and rigidity, and additionally, it has such a problem that the moldability thereof is bad because of its low flowability.
Of the above-mentioned thermoplastic resins, the polyamide resin is relatively well balanced between the characteristics. Generally used as the polyamide resin is an aliphatic polyamide, but this aliphatic polyamide has a high water absorption rate. Therefore, a connector formed from this aliphatic polyamide varies in its dimension, electrical resistance value, etc. when the connector absorbs water. Especially when the connector is warped, the connector is unable to be connected with the device.
By the way, an aromatic polyamide is known as a polyamide other than the aliphatic polyamide. The aromatic polyamide is obtained from an aromatic dicarboxylic acid as a dicarboxylic acid component and diamine and subjecting this aromatic dicarboxylic acid and diamine to polycondensation reaction.
The aromatic polyamide has a low water absorption rate differently from the aliphatic polyamide, and hence the above-mentioned problems such as decrease of dimensional accuracy and change of electrical resistance value occurring associated with the water absorption of the connector can be solved by using the aromatic polyamide.
However, as a result of further studies on the connector formed from the aromatic polyamide in more detail, the followings have been found. That is, when the connector is exposed to a high temperature, the aromatic polyamide is sometimes thermally deteriorated, and this thermal deterioration of the aromatic polyamide causes lowering of toughness of the connector. The connector thus lowered in toughness becomes poor in stretchability, and thereby the connector is hardly connected smoothly with the device.
Particularly in these years, electrical parts such as connectors are often incorporated into a device by soldering them through an infrared reflow method. If the connector is lowered in toughness by the heat of the infrared reflowing, reduction of workability in the assembly operation of the device or lowering of durability of the device is induced. Further, especially when the connector is used under such conditions that heating and cooling are repeatedly carried out, for example, under conditions within an automotive engine room, the toughness of the connector is easily reduced.
Japanese Patent Laid-Open Publication No. 60(1985)-44362 by the present applicant describes a composition of an aromatic polyamide having improved toughness. Concretely, the composition described in this publication contains the aromatic polyamide and a modified .alpha.-olefin elastic polymer.
In the above publication, studies on heat resistance required for engineering plastic products formed from the polyamide composition by a conventional melt molding method are disclosed, but there is not taken into account any property required for the case where a product made of the polyamide composition is exposed to an extremely high temperature as in the case of a connector of automobile. Accordingly, in order to improve reliability of connectors, resin molded products should be further improved in the long-term heat resistance.
For improving heat resistance of polyamide, there is known a method of adding various stabilizers to the polyamide, as well as the method of adding other resins to the polyamide. For example, Japanese Patent Laid-Open Publications No. 2(1990)-212533, No. 2(1990)-214752, No. 2(1990)-173059 and No. 62(1987)-273256 disclose a polyamide resin composition comprising a specific phenol type stabilizer, a specific sulfur type stabilizer and a specific phosphorus type stabilizer and an aliphatic polyamide such as polyamide 66 or polyamide(.epsilon.-caprolactam)/66. The aliphatic polyamide is used as the polyamide and the melting point of the aliphatic polyamide is much lower than that of the aromatic polyamide. Therefore, molded products formed from the compositions comprising the aliphatic polyamide and the stabilizers as described in the above publications exhibit excellent properties. However, if the aromatic polyamide is used instead of the aliphatic polyamide in the composition, foaming of the stabilizers is brought about during the preparation of a composition or the molding procedure of the composition, since the melting point of the aromatic polyamide is high.
Japanese Patent Laid-Open Publication No. 57(1982)-123254 discloses a composition comprising a polyamide, a specific phenol type stabilizer, a specific sulfur type stabilizer and a copper compound. In this composition, the copper compound is used as an essential component to exhibit sufficient thermal aging resistance, and hence the composition can be enhanced in heat stability when the copper compound is used in combination with the specific phenol type stabilizer and the specific sulfur type stabilizer. However, the composition sometimes suffers evil effects of metal caused by the copper compound which is added as the stabilizer. Especially when the composition is contaminated with metallic copper liberated from the copper compound, electrical properties of the resin sometimes vary, and the resin having such variability of electrical properties should not be used as a connector. Further, this resin composition also has such a problem that foaming is brought about during the preparation of the composition or the molding procedure thereof, similarly to the above-mentioned case. In other words, formulation of stabilizers having been conventionally applied to the aliphatic polyamide is not always satisfactory for the aromatic polyamide.